In order to operate rotating electrical machinery, such as motors and generators, at voltages in excess of an approximate 6 KV., AC., rating, it is necessary to prevent slot discharges from occurring between the outside of the coil insulation and the iron core laminations. Otherwise, corona damage will progressively erode the ground wall coil insulation. It is customary to help solve this problem by applying a linear, conducting, carbon black filled paint or tape layer on the outside of the mica tape or mica paper insulated coil at the slot portion of the coil. To prevent corona at the ends of the slots, a much higher resistivity, non-linear stress grading paint or tape layer is applied, with sufficient overlap of the end portion of the linear slot coating, to provide good electrical contact, as developed in the late 1950's by Virsberg et al., U.S. Pat. No. 3,066,180.
Virsberg et al. utilized a brushable, shrinkable paint composition, consisting of a base varnish of either oil-modified alkyd resin, polyurethane resin, or standard epoxy resin, incorporating from 10 wt. % to 65 wt. % of high-resistivity silicon carbide particles. The epoxy resins were of the glycidyl ether bisphenol A, bisphenol F, or novolac types, made with phenol and epichlorohydrin, which could be modified by addition of acidic polyesters. These coatings were applied, as a paint or coated tape, to in-place, cured, coil insulation systems, of up to about a 16 KV. rating, and then heated at up to about 60.degree. C., for about 6 hours, to cure the coating. The use of standard epoxy resins in the composition could, however, present tracking problems at higher KV. ratings.
In the early 1960's, other non-linear stress grading systems were taught by Berg et al., in U.S. Pat. No. 3,210,461, where the paint consisted of 6 parts by weight of SiC containing from 0.5 wt. % to 4 wt. % carbon, 1 part by weight of chlorofluorocarbon resin, and 4 parts by weight toluene. These coatings were also applied, as a paint, to in-place, cured, coil insulation systems of up to about a 20 KV. rating, and then heated to cure the coating. This non-linear grading paint solved corona problems for systems up to about a 20 KV. rating, but the resin was very expensive and not readily available, and it provided somewhat brittle coatings.
For higher KV. rated systems, a complicated system of precisely positioned polyester or epoxy resin strips, containing powdered barium titanate, have been embedded within the coil insulation at the ends of the slots, as taught by Philofsky et al., in U.S. Pat. No. 3,823,334. These high dielectric strips are arranged so that predetermined intermediate voltage belts control the electric field within the insulation, and the surface stress does not reach too large a value. This system requires a high measure of quality control.
Recently, rather than first insulating the mica tape or mica paper wrapped coils with, for example an anhydride cured epoxy resin system, followed by curing the insulating resin, then applying and curing stress coatings, and then finally inserting the coils into stator slots; post-impregnation insulating processes have been developed. In the post-impregnation insulating process, after the conducting tape layer and semi-conducting grading paint layer have been applied to mica tape or mica paper wrapped coils, the coils inserted into stator slots, and the motor or other electrical apparatus low temperature prebaked at about 115.degree. C. for 2 hours; the entire apparatus is then placed in a large tank, and insulated with a reactive impregnating resin composition by a vacuum pressure impregnation process (V.P.I.). The V.P.I. resin composition is then baked at 150.degree. C. to 180.degree. C. to cure. This post-impregnation and hot baking with the grading paint already in-place, created a host of new problems.
During the prebake operation, the temperature must be kept below about 130.degree. C., or harm will be caused to other insulating materials, mica tape backing and some tying materials used in the assembled electrical apparatus. High prebake temperatures may also cure the small amount of catalyzed resin bond in the mica tape or mica paper, and interfere with subsequent resin impregnation. However, at prebake temperatures below about 130.degree. C., most resins heretofore used to bind the SiC in the semiconducting grading paint are not completely cured, unless extended time periods are used.
Subsequent contact with reactive, diluted, catalyzed, V.P.I. resins, particularly epoxy-acid anhydride V.P.I. insulating resin compositions, especially during V.P.I. resin cure at 150.degree. C. to 180.degree. C., can cause degradation of the semi-conducting grading composition. It is thought, that the highly polar resins of the grading composition can be chemically attacked by the combination of highly polar reactive diluent and acid anhydride in the reactive V.P.I. resin composition, which can cause blistering, flaking, and other very serious deleterious electrical changes. The term "highly polar insulating resin" is herein defined to mean V.P.I. resins containing reactive diluents and/or acid anhydrides.
As shown by A. J. Stock, in 1971 EDN Bulletin, Cahners Publication, "Conductive Coatings Compared", May 1, 1971, pp. 1-5, most common phenolic, epoxy and silicone resins, while having high maximum service temperatures, also have 1 hour cure temperatures of over about 150.degree. C. Alkyd resins, while having lower 1 hour cure temperatures, are generally shown to also have lower maximum service temperatures.
What is needed is a SiC-resin grading system that will have a high maximum service temperature, and will both cure at a temperature of up to 130.degree. C., preferably within a 21/2 hour span, and will also be completely resistant in the cured state to chemical attack by contacting, reactive V.P.I. resin compositions containing catalysts, acid components such as acid anhydride curing agents, reactive diluents and the like, at room temperature, and also at V.P.I. resin cure temperatures of up to 180.degree. C. The non-linear grading composition must also be simple, inexpensive, arc-track resistant, flexible so as to resist cracking due to vibration, and compatible with mica and SiC and other components of the electrical apparatus, and have an adequate shelf life so that it is commercially useful. This Si-C grading coating is needed for systems where the insulation resin is not applied and cured before Si-C grading coating application.